On Thu, Sep 24, 2015 at 2:17 PM, Tier Nolan via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

>
> Is there actually a requirement for the new message?  New nodes could just
> unilaterally switch to sending headers and current nodes would be
> compatible.
>

I don't believe that unilaterally switching to headers announcements would
work for all network participants -- both for users running older Bitcoin
Core versions (anything before 0.10, which I believe all ignore headers
messages) and for non-Bitcoin Core software that participates on the
network (which may ignore headers messages too, I'm not sure what all is
out there).

Even for Bitcoin Core versions 0.10 and 0.11, which process headers and use
them to determine what blocks to download, the block fetching logic is not
optimized for new block announcements via headers messages.  Part of what
is implemented in the pull request is direct fetching of blocks upon
receiving a headers message; nodes that don't implement block downloading
in response to headers announcements should continue to receive inv's, I
think -- hence this p2p message to opt-in to the new behavior.